Re: security for a 72 blazer
I take it the blazer has an electric fuel pump?
Chances are no one will want to steal it but if they want it an alarm isn't going to deter them much. You'd have to get an expensive one that you either attach to the glass or can "magically" sense glass breaking.
If you do a starter cut off switch you're going to have to do it at the solenoid. Otherwise, with how simple the wiring is in that Blazer they'll be able to spot where you put a switch in under the dash.
If you want power door locks for keyless entry then you can buy a kit from GM for $20. I don't know the part number but it comes in a white box 1"x3"x10". It has the glorified rack and pinion device, a rod, a clip, and a mounting bar. It's the standard GM type that reverses polarity in order to make it lock and unlock. All you have to do is hook it up how the alarm/keyless entry thing says. I've stuck them in a lot of vehicles that didn't have power door locks so the keyless entry/remote start system could unlock the driver door.
If you want to put power door locks and windows like from the factory that'd be a nightmare. You'd have to find new window regulators, lock devices, and door pads.